Concept : In the Dissolve mode, Photoshop will randomly change the layer pixels to 0% opacity and other will remain to 100%. This effect can be checked when the layer is set to the opacity less than 100%.
1. Open Photoshop and open new file with white background.
2. Make sure to keep RGB mode or change the mode by Image > Mode > RGB color
3. Create new layer. Select Rectangular Marquee Tool. Draw rectangle. Fill it with red color. Deselect the selection with CTRL + D
4. Create another layer. Draw another rectangle that overlaps some portion of the red rectangle in the layer below. Fill it with green color. Deselect the selection with CTRL + D .
Keep the mode on Dissolve. You will not find any change in the image.
5. Change the opacity to 80% and check out the effect.
6. The green solid filled rectangle is changed into the dotted rectangle.